Directional drilling has proven useful in facilitating production of fluid, e.g., hydrocarbon-based fluid, from a variety of reservoirs. In many such operations, a vertical wellbore is drilled, and casing is deployed in the vertical wellbore. One or more windows are then milled through the casing to enable drilling of lateral wellbores. Each window formed through the casing is large enough to allow passage of components, e.g., passage of a bottomhole assembly used for drilling the lateral wellbore and of a liner for lining the lateral wellbore. The bottomhole assembly may comprise a variety of drilling systems, such as point-the-bit and push-the-bit rotary drilling systems.
In some operations, the bottomhole assembly is relatively long and lacking in flexibility, which can create difficulty in forming a suitable casing window for passage of the bottomhole assembly. Formation of casing windows, particularly longer and/or larger casing windows to better accommodate longer and stiffer bottomhole assemblies, requires substantial removal of material. Existing whipstock and mill designs tend to create substantial loading on specific cutters or cutter regions of the mill and this can lead to excessive wear and reduction in cutting efficiency, particularly when cutting larger casing windows.